After reading the other posts/threads. I still stand by my origenal comment. Not all engine guards are built the same way or by the same manufacturer, so it can't be trusted that they are vacuum sealed and clean etc. Same with the frame. Most that I've seen are just stock tubing with end caps welded on, no vacuum presure. Watch any bike building show on TV and you'll see what I'm talking about. So the tubing cannot be guarenteed to be clean on the inside.
If they were vacuum sealed it would be hard as hell to weld the darn things. How would they keep the presure in the tube higher then the outside presure while they welded it? Its not like they could hook up a rubber hose or something. They would have to be welded in a room with a high vacuum presure etc, and I doubt a manufacturer is going to go through the expense of creating a vacuum presure controlled welding room just to make engine guards. That would be just silly.
One quick way you can tell if an engine guard was vacume sealed when it was manufactured is when/if you drill your holes in it for the oil inlet/outlet. If once you brake through the plate with the drill you don't hear a hissing sound or pop, it was _not_ vacuum sealed. If it _is_ vacuum sealed the air presure in the tube will be higher then the outside air presure, and will make an audible sound when the seal is broken and the presure between the two equalizes. Thats simple physics.
Also, vacuum presure doesn't prevent moisture from being in the air thats in the tube. And everyone knows moisture creates rust....
Just my zero cents (after taxes)
--Rob